1. Field of the Invention
The present invention relates generally to non-invasive and implantable (i.e., invasive) methods, devices and system for assessing heart failure status using morphology of a signal representative of arterial pulse pressure. Specific embodiments of the present invention relate to assessing heart failure status based on the morphology of a plethysmography signal.
2. Related Art
Heart failure (HF) is a pathophysiologic state in which an abnormality of myocardial function inhibits the ventricles from delivering adequate quantities of blood to metabolizing tissues at rest or during activity. It results not only from a decrease in intrinsic systolic contractility and/or diastolic relaxation of the myocardium but also from alterations in the pulmonary and peripheral circulations as well. HF can develop from a variety of different causes. Coronary artery disease, hypertension, and idiopathic cardiomyopathy are common risk factors for HF. Acute conditions that may result in HF include acute myocardial infarction (AMI), arrhythmias, pulmonary embolism, sepsis, and acute myocardial ischemia. Gradual development of HF may be caused by liver or renal disease, primary cardiomyopathy, cardiac valve disease, anemia, bacterial endocarditis, viral myocarditis, thyrotoxicosis, chemotherapy, excessive dietary sodium intake, and ethanol abuse. Drugs can also worsen HF. Drugs that may cause fluid retention, such as nonsteroidal anti-inflammatory drugs (NSAIDs), steroids, hormones, antihypertensives (e.g., hydralazine, nifedipine), sodium-containing drugs (e.g., carbenicillin disodium), and lithium may cause congestion. Beta blockers, antiarrhythmics (e.g., disopyramide, flecainide, amiodarone, sotalol), tricyclic antidepressants, and certain calcium channel blockers (e.g., diltiazem, nifedipine, verapamil) have negative inotropic effects and further decrease contractility in an already depressed heart. Direct cardiac toxins (e.g., amphetamines, cocaine, daunomycin, doxorubicin, ethanol) also can worsen or induce HF.
When the heart fails as a pump and cardiac output (the volume of blood pumped out of the ventricle per unit of time) decreases, a complex scheme of compensatory mechanisms to raise and maintain perfusion to vital organs. These compensatory mechanisms include increased preload (volume and pressure or myocardial fiber length of the ventricle prior to contraction, i.e., end of diastole), increased afterload (vascular resistance), ventricular hypertrophy (increased muscle mass) and dilatation, activation of the sympathetic nervous system (SNS), and activation of the renin-angiotensin-aldosterone (RAA) system.
Although initially beneficial for maintaining perfusion, these compensatory mechanisms are ultimately associated with further pump dysfunction. In effect, the consequence of activating the compensatory systems is a worsening of the HF. This is often referred to as the xe2x80x9cvicious cycle of HFxe2x80x9d. Without therapeutic intervention, some of the compensatory mechanisms continue to be activated, ultimately resulting in a reduced cardiac output and a worsening of the patient""s HF symptoms. It becomes apparent why one goal in the treatment of HF is to interrupt this vicious cycle as soon as possible. Accordingly, there is a need for detecting HF as early as possible.
Prior attempts to detect HF symptoms require a large amount of interaction by the patient. For example, U.S. Pat. No. 6,080,106 (Lloyd, et al.) describes a patient interface system with a scale. The patient interface system described in the ""106 patent includes a patient data input means having both a scale and a question and answer means. The question and answer means presents the patient with one or more questions related to the patient""s health status and records the patient""s answers to the questions. Example questions include: (1) xe2x80x9cWere you tired during the day?xe2x80x9d; (2) xe2x80x9cOn a scale of 1 to 5, 5 being most, how tired were you in the middle of the day?xe2x80x9d; (3) xe2x80x9cDid you cough during the night?xe2x80x9d; (4) xe2x80x9cDid you need an extra pillow to sleep?xe2x80x9d; (5) xe2x80x9cAre your shoes tighter than usual?xe2x80x9d; (6) xe2x80x9cDid you exercise today?xe2x80x9d; and the like. In operation, the patient steps onto a scale, which automatically activates a processor that compares the weight measured by the scale with the minimum and maximum weights stored in a memory. The measured weight and deviation (if any) from the target weight is displayed on a visual display, and is stored for later transmission to a monitoring staff. The question and answer means then presents questions selected by the patient""s physician, designed to elicit details about the patient""s condition. The patient responds by pressing a button that corresponds to the desired answer, or, optionally, the patient simply speaks his or her responses into microphone. Once the series of questions and answers is completed, a processor transmits the measured data and patient""s answers to the monitoring staff via modem. While connected to the monitoring staffs computer, the answers and data are examined by the monitoring staff (or compared immediately by the monitoring staff s computer), and new questions, target values, and minimum/maximum values are downloaded to the processor. In this manner, cardiac associated diseases, such as HF, can be remotely monitored. A problem with the system of the ""106 patent is that it requires a large amount of interaction by the patient. Minimally, the system requires that the patient step on a scale and answer one or more questions. This requires that the patient has the time and the initiative to performs these steps. This also requires that the patient remembers to perform these steps. Additionally, such questions and answers are very subjective, resulting in a very subjective monitoring of HF.
Some of these limitations have been addressed by the development of an implantable system that monitors hemodynamic status (Medtronic Chronicle, Medtronic, Inc., Minneapolis, Minn.). While this system potentially avoids the need for active patient participation, it relies on an intravascular sensor placed in the right ventricle of the heart. This approach is consistent with the prior art for implantable hemodynamic status monitoring, which has to date focused on intravascular or intramyocardial instrumentation. Examples include U.S. Pat. No. 5,454,838 in which Vallana et al. teach placement of a sensor on the myocardial wall using an intravascular approach. In U.S. Pat. No. 5,496,351, Plicchi et al. propose placing a sensor within the myocardial wall. Mortazavi in U.S. Pat. No. 5,040,538 and Cohen et al. in U.S. Pat. No. 4,815,469 describe placement of an optical sensor within the right ventricle. In the context of hemodynamic assessment for arrhythmia discrimination, Cohen and Liem (Circ., 1990, 82:394-406) study the effectiveness of a pressure transducer placed in the right ventricle. Clearly, powerful information about hemodynamic status can be obtained using intravascular instrumentation. However, intravascular or intramyocardial instrumentation carries significant risks to the patient, including increased perioperative morbidity and mortality, and increased long-term risks such as stroke and pulmonary embolism. Furthermore, intravascular instrumentation can only be performed by extensively trained specialists, thereby limiting the availability of qualified physicians capable of implanting the device, and increasing the cost of the procedure. Finally, because of the added patient risks and greater physical demands of an intravascular environment, the intravascular placement of the sensor increases the cost of development, manufacturing, clinical trials, and regulatory approval.
There is a need for methods, devices and systems that can access the HF status of patients with minimal or no interaction by the patient. Preferably, such methods, devices and systems do not rely on subjective information. Further, it would be beneficial if such methods, devices and systems can be as inexpensive and safe as possible. It would also be beneficial if such methods, devices and systems can be implemented into methods, devices and systems that are already used to monitor the heart for other reasons. In addition, to reduce cost and increase safety, it would be desirable to use monitoring methods and devices that do not require intravascular instrumentation.
The present invention is directed to methods for assessing heart failure (HF) status, and monitoring devices and systems that assess HF status. More specifically, the present invention relates to assessing HF status based on morphology of a signal representative of arterial pulse pressure. Specific embodiments of the present invention relate to assessing heart failure status based on the morphology of a plethysmography signal.
An embodiment of the present invention includes producing a plethysmography signal that is representative of arterial pulse pressure. HF status is then assessed based on the frequency characteristics of the plethysmography signal. Each cardiac cycle of the plethysmography signal includes a primary pulse, a secondary pulse, and a dicrotic notch that separates the primary and secondary pulses. The height of and area under these pulses change when a HF exacerbation is developing. This enables HF assessment to be based on the frequency characteristics of a plethysmography signal.
The plethysmography signal can be produced by transmitting light from a light source (e.g., toward a capillary bed) and receiving a portion of the light transmitted from the light source at a light detector. The portion of light received at the light detector has an associated detected light intensity that is directly representative of blood volume, which is indirectly representative of the arterial pulse pressure. The plethysmography signal is then produced based on the received portion of light. The light source and the light detector can be arranged in a transmission or reflection configuration.
In an alternative embodiment, the plethysmography signal is produced by transmitting light from a light source (e.g., toward a capillary bed), wherein an intensity of the transmitted light is based on a light control signal. A portion of the transmitted light is then received at a light detector, the received portion having an associated detected light intensity. A feedback signal is produced based on the received portion of light, wherein the feedback signal is indicative of the detected light intensity. The feedback signal is then compared to a reference signal to produce a comparison signal, which is used to adjust the light control signal. The plethysmography signal is then produced based on the comparison signal and/or the light control signal.
According to an embodiment of the present invention, an alert indicator is triggered based on the shape of the plethysmography signal.
In an embodiment of the present invention, a chronically implantable sensor is used to produce a signal that is representative of arterial pulse pressure. HF status is then assessed based on the frequency characteristics of the signal that is representative of arterial pulse pressure. The chronically implantable sensor can be an extravascular sensor that includes a light source and a light detector, and the signal can be a plethysmography signal. Alternatively, the chronically implantable sensor can be a intra-arterial sensor, such as a pressure sensor. The intra-arterial sensor should be implanted in an appropriate location, such as the pulmonary artery, so that a signal including the desired dicrotic characteristics is produced. The values mentioned above can be determined based on the signal produced using the chronically implanted sensor. HF status can then be assessed based on these values.